Chemistry Class- AAS-Flameless Instrument.ppt

AAS Flameless

1. 1
Atomic Absorption
Spectroscopy
Prof Drs Mudasir, M.Eng.,
Ph.D
Chemistry Department, UGM

2. 2
Atomic Absorption Spectroscopy
•AAS is commonly used for metal analysis
•A solution of a metal compound is sprayed
into a flame and vaporises
•The metal atoms absorb light of a specific
frequency, and the amount of light
absorbed is a direct measure of the number
of atoms of the metal in the solution

3. 3
Atomic Absorption Spectroscopy:
•Developed by Alan Walsh (below) of the
CSIRO in early 1950s.

4. 4
Atomic Absorption Spectroscopy
•Uses absorption of light to measure the
concentration of gas-phase atoms.
•Since samples are usually liquids or solids, the
analyte atoms must be vapourised in a flame
(or graphite furnace).

5. 5
Absorption and Emission
Ground State
Excited States
Absorption Emission
Multiple
Transitions

6. 6
Atomic Absorption Spectroscopy
•The analyte concentration is determined
from the amount of absorption.

7. 7
Atomic Absorption Spectroscopy
•The analyte concentration is determined from
the amount of absorption.

8. 8
Atomic Absorption Spectroscopy
•It is possible to measure the concentration
of an absorbing species in a sample by
applying the Beer-Lambert Law:
Abslog
I
I
o






Abscb
 = extinction coefficient

9. 9
Atomic Absorption Spectroscopy
•But what if  is unknown?
•Concentration measurements can be
made from a working curve after
calibrating the instrument with standards
of known concentration.

10. 10
AAS - Calibration Curve
•The instrument is calibrated before use by testing
the absorbance with solutions of known
concentration.
•Consider that you wanted to test the sodium
content of bottled water.
•The following data was collected using solutions of
sodium chloride of known concentration
Concentration (ppm) 2 4 6 8
Absorbance 0.180.380.520.76

11. 11
Calibration Curve for Sodium
Concentration (ppm)
A
b
s
o
r
b
a
n
c
e
2 4 6 8
0.2
0.4
0.6
0.8
1.0

12. 12
Use of Calibration curve to determine
sodium concentration {sample absorbance =
0.65}
Concentration (ppm)
A
b
s
o
r
b
a
n
c
e
2 4 6 8
0.2
0.4
0.6
0.8
1.0
Concentration
Na
+
= 7.3ppm

13. 13
Hollow-Cathode Lamps
•The electric discharge ionises rare gas
(Ne or Ar usually) atoms, which in turn,
are accelerated into the cathode and
sputter metal atoms into the gas phase.

14. 14
Hollow-Cathode Lamps

15. 15
Hollow-Cathode Spectrum
Harris Fig. 21-
3:
Steel hollow-
cathode

16. 16
Atomisation
•Atomic Absorption Spectroscopy (AAS)
requires that the analyte atoms be in the gas
phase.
•Vapourisation is usually performed by:
–Flames
–Furnaces
–Plasmas

17. 17
Flame Atomisation
Harris
Fig 21-4(a)

18. 18
Flame Atomisation
•Degree of atomisation is temperature
dependent.
•Vary flame temperature by fuel/oxidant
mixture.
Fuel Oxidant Temperature (K)
Acetylene Air 2,400 - 2,700
AcetyleneNitrous Oxide2,900 - 3,100
Acetylene Oxygen 3,300 - 3,400
Hydrogen Air 2,300 - 2,400
Hydrogen Oxygen 2,800 - 3,000
Cyanogen Oxygen 4,800

19. 19
Furnaces

20. 20
Furnaces

21. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy

22. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Cold Vapor and Hydride generation Atomic Absorption
•FAAS and GFAAS can measure most elements
•Cannot measure: mercury, selenium and arsenic
•All too volatile to be measured by flame or furnace techniques

23. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Cold Vapor and Hydride generation
Atomic Absorption
Cold Vapor Atomic Absorption
(CVAA) Spectroscopy for Hg
•Free mercury atoms exist at room
temperature, no requirement for
heating
•Sample may contain Hg
0
, Hg
2
2+
or
Hg
2+
•In CVAA Hg is chemically reduced
to the atomic state by reaction with
a strong reducing agent (e.g.
SnCl
2
or NaBH
4
) in a reaction flask
•Hg is then carried via gas stream
to the absorption cell

24. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Cold Vapor and Hydride generation Atomic Absorption
Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se
•AsH
3 and SeH
3 generated by reaction samples containing As and Se with NaBH
4
•Uses same setup as FAAS except it switches nebulizer for the hydride generation
module

25. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Cold Vapor and Hydride generation Atomic Absorption
Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se
•Sample is reacted in the external hydride generator with reducing agent (NaBH4)
•Hydride generated is then carried via inert gas to the sample cell in the light path of
the FAAS
•Unlike CVAA product is not free atoms but AsH
3
/ SeH
3
which are not measurable
•Sample cell must be heated to dissociate the hydride into free atoms (As
0
) and Se
0
)
•Higher sampling efficiency leads to
lower detection limits:
ppb vs ppm for regular FAAS and
GFAAS

26. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Inductively Coupled Plasma Atomic Emission (ICP-OES)

27. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Inductively Coupled Plasma Atomic Emission (ICP-OES)
•Sample is nebulized and entrained in the flow of plasma support gas (Ar)

28. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Inductively Coupled Plasma Atomic Emission (ICP-OES)
•Plasma torch inner tube contains the sample aerosol and Ar support gas
•Radio frequency generator produces a magnetic field which sets up
an oscillating current in the ions and electrons of the support gas (Ar)
•Produces high temperatures (up to 10,000 K)

29. Atomic Spectroscopy for Metal Analysis
Instruments for Atomic Spectroscopy
Inductively Coupled Plasma Atomic Emission (ICP-OES)
•Atomizes the sample and promotes atomic and ionic transitions which are observable
at UV and visible wavelengths
•Excited atoms and ions emit their characteristic radiation, which are collected by a
device that sorts the radiation by wavelength
•Intensity of the emission is detected and turned into a signal that is output as
concentration

30. Atomic Spectroscopy for Metal Analysis
Selection of the Proper Atomic Spectroscopic Techniques
Comparison of Detection Limits and Working Range
•Low detection limit is essential for trace analysis
•Without low level capability – sample pre-concentration is required
FAAS > ICP-OES > HGAAS > GFAAS > ICP-MS

31. Atomic Spectroscopy for Metal Analysis
Selection of the Proper Atomic Spectroscopic Techniques
Comparison of Detection Limits and Working Range

32. Atomic Spectroscopy for Metal Analysis
Selection of the Proper Atomic Spectroscopic Techniques
Comparison of Interferences and Other Considerations
Interference
•Spectral: in spectroscopy, interference occurs when another emission line (e.g. from
other elements in the sample) is close to the emitted line of the test element and is
not resolved by the monochromator
•Chemical: formation of undesired species during atomization
•Physical: variation of instrument parameters such as uptake in the burner and
atomization efficiency (gas flow rate, sample viscosity etc.)

33. Atomic Spectroscopy for Metal Analysis
Selection of the Proper Atomic Spectroscopic Techniques
Comparison of Interferences and Other Considerations
Other Considerations
•Cost:
FAAS < GFAAS > ICP-OES << ICP-MS

34. Atomic Spectroscopy for Metal Analysis
Practical Tips to Sampling
Erroneous Data and Methods of Compensation
•Erroneous results arise due to one or more of the sources of interference described
above
•Compensation methods: background correction, higher temperatures, release agent,
alternative wavelength, internal standard, matrix spike, etc.
•e.g. Chemical interferences
–Refractory salts e.g. PO
4
3-
, SO
4
2-
and silicate ion
e.g. Ca
2+
forms refractory insoluble Ca
3(PO
4)
2
–Add release agent (10% lanthanum solution or EDTA)
•Complex solutions (matrix) require method of standard additions
–Add small volumes higher concentration standards (change in volume is negligable)
–Graph of concentration vs. absorbance
–Concentration of sample is x-intercept
–Overcomes problem of matrix effects

35. Atomic Spectroscopy for Metal Analysis
Practical Tips to Sampling
Erroneous Data and Methods of Compensation
•Simple solutions (e.g. water) use standard curve technique to find unknown
concentration
•Complex solutions (matrix) require method of standard additions
–Add small volumes higher concentration standards (change in volume is
considered negligible)
–Graph of concentration vs. absorbance
–Concentration of sample is x-intercept
–Overcomes problem of matrix effects

36. Teknik-Teknik Analisis
Ada tiga teknik yang biasa dipakai dalam analisis secara spektometri. Ketiga
teknik tersebut adalah :
•Metoda Standard Tunggal
Metoda ini sangat praktis karena hanya menggunakan satu larutan standard yang
telah diketahui konsentrasinya (C
std
). Selanjutnya absorbsi larutan standard (Astd)
dan absorbsi larutsn sampel (A
smp
) diukur dengan Spektrofotometri. Dari hukum
Beer diperoleh :
A
std
= .b.C
std
A
smp
= .b.C
smp
.b = A
std/ C
std .b = A
smp/ C
smp
sehingga,
A
std
/C
std
= A
smp
/C
smp
C
smp
= (A
smp
/A
std
) x C
std
Dengan mengukur Absorbansi larutan sampel dan standar, konsentrasi larutan
sampel dapat dihitung.
•Metode Kurva Kalibrasi
Dalam metode ini dibuat suatu seri larutan standar dengan berbagai konsentrasi
dan absorbansi dari larutan tersebut diukur dengan AAS. Langkah selanjutnya
adalah membuat grafik antara konsentrasi (C) dengan Absorbansi (A) yang akan
merupakan garis lurus melewati titik nol dengan slope = .b atau slope = a.b.
Konsentrasi larutan sampel dapat dicari setelah absorbansi larutan sampel diukur
dan diintrapolasi ke dalam kurva kalibrasi atau dimasukkan ke dalam pers. garis
lurus yang diperoleh dengan menggunakan program regresi linear pada kurva
kalibrasi

37. Teknik-Teknik Analisis
•Metoda Adisi Standard
Metoda ini dipakai secara luas karena mampu meminimalkan kesalahan yang
disebabkan oleh perbedaan kondisi lingkungan (matriks) sampel dan standard. Dalam
metoda ini dua atau lebih sejumlah volume tertentu dari sampel dipindahkan ke dalam
labu takar. Satu larutan diencerkan sampai volume tertentu, kemudian diukur
absorbansinya tanpa ditambah dengan zat standard, sedangkan larutan yang lain
sebelum diukur absorbansinya ditambah terlebih dulu dengan sejumlah tertentu larutan
standard dan diencerkan seperti pada larutan yang pertama. Menurut hukum Beer akan
berlaku hal-hal berikut :
A
x
= k.C
x
; A
T
= k(C
s
+ C
x
)
dimana,
C
x
= konsentrasi zat sampel
C
s = konsentrasi zat standar yang ditambahkan ke larutan sampel
A
x
= Absorbansi zat sampel (tanpa penambahan zat standar)
A
T
= Absoebansi zat sampel + zat standar
Jika kedua persamaan diatas digabung, akan diperoleh:
C
x
= C
s
x {A
x
/(A
T
-A
x
)}
Konsentrasi zat dalam sampel (C
x) dapat dihitung dengan mengukur A
x dan A
T dengan
spektrofotometer. Jika dibuat suatu seri penambahan zat standar dapat pula dibuat
suatu grafik antara A
T lawan C
s, garis lurus yang diperoleh diekstrapolasi ke A
T = 0,
sehingga diperoleh:
C
x
= C
s
x {A
x
/(0-A
x
)} ; C
x
= C
s
x (A
x
/-A
x
)
C
x = C
s x (-1) atau C
x = -Cs

38. Kurva Kalibrasi dan Adisi StandarKurva Kalibrasi dan Adisi Standar

Abs AT

Ax A=a.b.C



Cx ppm standar -Cx 0 Cs-1 Cs-2 ppm tambahan

Gambar III.9 Kurva kalibrasi (kiri) dan kurva adisi standar (kanan) dalam analisis
secara spektrometri.

39. BEBERAPA CONTOH APLIKASI DARI AAS
•Timah hitam dalam darah
Timah hitam(Pb) dalam darah dapat ditentukan dengan AAS. Mula-mula
darah ditreatment dengan Trichlorocetic Acid (TCA) untuk
mengendapkan protein, dan kemudian di “centrifuge” (diputar). pH dari
filtrat dibuat menjadi tiga, kemudian ditambahkan 1 ml APDC dan Pb
diekstrak dengan MIBK sebagai chelate Pb(APDC)
2. Hasil ekstraksi
kemudian diukur absorbansi Pbnya dengan AAS menggunakan nyala
udara asetilen . Batas deteksi prosedur ini adalah 0,1 ppm Pb dalam
darah sedangkan kandungan normal Pbdalam darah adalah 0,3 – 0,4
ppm. APDC : Amonium pirolidin dithiokarbamat; MIBK : Metil Isobutil
Keton.
•Seng (Zn) dalam Tumbuhan
Zn adalah unsur runut esensial dalam tumbuhan. Beberapa gram
sampel tumbuhan yang telah dikeringkan dan dan digerus diabukan
dalam krus silika selama semalam pada suhu 500 oC. Abu yang
diperoleh dilarutkan dengan 6 M HCl dan dikeringkan perlahan-lahan
dengan steambath. Endapan yang diperoleh dilarutakan ke dalam HCl
encer kemudian disaring. Selanjutnya absorbansi Zn dalam larutan ini
diukur dengan AAS.

40. Aplikasi AAS ……
•Tembaga dalam air laut
Beberapa elemen dalam air laut berada dalam tingkat ppb atau lebih rendah.
Oleh karenanya harus dipekatkan terlebih dahulu sebelum dianalisis misalnya
Cu dalam air laut (1-25 ppb). Cu dalam air laut dapat ditentukan dengan AAS
setelah diekstrak sebagai kompleks APDC pada pH larutan kira-kira 3. Pelarut
ekstraksi yang dipakai adalah metil-n-amil keton yang kelarutannya dalam air
cukup rendah.
•Berilium dalam partikel udara
Partikel dikumpulkan dengan kertas cellulose asetat membrane (misal e.g.,
Milipore). Filter ini kemudian diabukan pada suhu rendah. Abu yang diperoleh
dilarutkan dalam Hcl encer kemudian diukur dengan AAS. Hasil dilaporkan
dalam mg/meterkubik udara.
•Na, K, Mg, dan Ca dalam semen
Sampel didekomposisi dengan 4 M HCl dan diuapkan sampai kering, endapan
yang diperoleh dilarutkan kembali dengan 4 M HCl dan diencerkan. Larutan
yang diperoleh diukur absorbansinya untuk masing unsur di atas. Gangguan
dari phosphat, silikon, dan aluminium pada pengukuran absorbansi Ca dapat
dihilangkan dengan menambahkaaaan larutan strontium konsentrasi tinggi.

41. Question
A series of solutions is made up by adding 0.1, 0.2, 0.3, 0.4 and 0.5 mL of a
10 mg L
-1
lead standard to 100 mL aliquots of the unknown solution. The following
results were obtained:
Volume std. (mL) 00.1 0.2 0.3 0.4 0.5
Abs 0.27 0.37 0.53 0.65 0.75 0.88
Plot a calibration graph and determine the concentration of the unknown
Assuming constant volume of 100 mL, the concentration increase in the
5 solutions are 10, 20, 30, 40, and 50 μg L
-1
.
(e.g. 0.5 mL of 10 mg/L = 5 x 10
-3
mg in 100 mL = 5 μg/0.1 L = 50 μg L
-1
)
Absorbance = (0.01235 x conc) + 0.2694
Unknown = 21.8 g μL
-1
lead

42. Quantification